1. Field of the Invention
The present invention is related to a resonant protection kit for power supply and more particularly to the resonant protection kit used for avoiding over-voltage/over-current due to resonance phenomenon generated by a combined inductance/capacitor set of an electric power system, which will result in damage to the inductance/capacitor set, so as to achieve the protection of the inductance/capacitor set.
2. Description of the Related Art
In the electric power system most of electrical loads currently are inductance. And in this electric power system power delay occurs due to these inductive loads consuming power. Accordingly, a predetermined real power can be obtained by transmitting a greater current that results in reduction of transmission efficiency in a power distribution system and increasing the possibility of load regulation in its load terminal. In order to solve the above problem, power stations or users add a capacitor set or an automatic power regulator in power line to produce a necessarily inductive load nearby. Capacitance of a capacitor set loaded in a power system is about 25 to 35 percent of total capacitance of power system even as much as about 50 percent in some other power system, as indicated from previous studies.
Electric power devices, technique and facilities such as an Uninterruptible Power Supply, a converter, a rectifier, a frequency converter, and an inverter are widely applied in power distribution system. The load losses of these electric facilities are non-linear input characteristic and the input powers of these electric facilities are low factors. In general, voltage and current of waveform of power transmission and power distribution system are designed to operate under the condition of sine wave of constant-rate frequency. However, when a large number of such non-linear electric facilities are applied, these electric facilities produce different degree of harmonic current and unavailable power. A waveform of the voltage of the power system is distorted due to inputting harmonic current and unavailable power and the distorted waveform results in reduction of power quality. In electric facilities of power system, an inductance is usually connected in series with a capacitor being to be limited reactance or to form a filter. But the inductance and capacitor of these electric facility, however, is severely damaged by these harmonic currents.
In Japan for example, electric component is damaged about 65 percent for inductance and about 26 percent for capacitor in power system by the harmonic current, and the total damage exceed 91 percent. Consequently, the harmonic damaging to the inductance and the capacitor is severe problem.
In harmonic damage problem, damage's reason of an inductance/capacitor set may include over-voltage resulting in insulator damage, or over-current resulting in overheated damage. The voltage/current of all inductance/capacitor set are selectively determined at a normal rated value for operating in normal rated state so the inductance/capacitor set in this normal rated state should not be damaged by over-voltage or over-current. However, the over-voltage/over-current of the inductance/capacitor set may be frequently produced a resonance phenomenon between the main impedance of a power system and its branch impedance of the inductance/capacitor set. When a power distribution system has similar frequencies of harmonics, these harmonics are amplified over ten times that produces over-voltage/over-current and thus the inductance/capacitor set is burnt.
In order to avoid the over-voltage/over-current damaging to the inductance/capacitor set, in common application a power factor improved system or a passive power filter system is usually increased its capacitance grade to solve the resonance phenomenon damaging to the inductance/capacitor set. This solution is not only increase overall cost but also fails to absolutely solve the damage of the inductance/capacitor set. Moreover, increasing capacitance grade of the capacitor set is relative to increase the current grade of its capacitor. Thus the inductance is easily burnt by the resonance phenomenon if its normal rated current is inadequate. This is of a main problem that inductance damage led with about 65 percent, followed by 26 percent for the capacitor damage. Consequently, the previous solution is ineffective for the damage of the inductance/capacitor set caused by the resonance phenomenon.
Power capacitor's tolerance voltage and over-current rule according to IEC831 and IEC871 standards are followings: 1. Operative voltage is determined 110% mean square root of normal rated voltage, including various harmonic voltages (excluding system temporary state) for normal operation; 2. In order to match up system temporary state or abnormal operative conditions, the power capacitor is capable of allowing over-current in a short time according to IEC standard; 3. Operative current is determined 130% mean square root of normal rated current, including fundamental waveform currents and various harmonic currents for normal operation.
TABLE 1IEC standard for allowance time in transient over-voltage of capacitorMaximum allowable mean square root ofContinuous allow timenormal rated voltage (%)1min.130%(1.3 times)5min.120%(1.2 times)30min.115%(1.15 times)12hr.110%(1.1 times)
According to above IEC standard for tolerance voltage and over-current rule, electric facility (inductance/capacitor set) can avoid damage caused by over-voltage/over-current if it is cut off or changed its impedance characteristic within tolerance time or before over-current exceeding 130%.
The present invention intends to provide a resonant protection kit for power supply which comprises a protection module and senses over-voltage/over-current according to IEC standard, so as to cut off an electric facility from a power system or to adjust its impedance characteristic in such a way to mitigate and overcome the above problem.